To maximize the transmission capacity of an optical fiber transmission system, a single optical fiber may be used to carry multiple optical signals in what is called a wavelength division multiplex system (hereinafter a WDM system). Modern WDM systems have a high traffic capacity, for example, a capacity to carry 64 channels of 10 gigabits per second (hereinafter Gb/s). When an optical link is initially deployed, however, the link may be only partially loaded. Initially, only a few of the total number of potential channels may be used to carry information signals.
When the information signals are transmitted over long distances or between links of optical fiber cable, one or more amplifiers may be provided to compensate for signal attenuation. The amplifiers used in some WDM systems cannot easily be modified, and must be sized initially to support a fully loaded link (e.g., 64 channels, each channel carrying 10 Gb/s). The power per channel must be sufficient to provide an adequate signal to noise ratio in the presence of the amplified spontaneous emission noise from the amplifiers, necessitating a high amplifier total output power for systems with high fully-loaded capacity. The amplifiers are thus configured to provide an optical output signal at a nominal optical power. The nominal output power level is insensitive to the power at the input of the amplifier. As the amplifier input power varies over a wide range, the output power changes very little around this nominal output power level. Thus, when the optical link is fully loaded, each channel is amplified to a substantially equal optical output power. If the initially deployed system uses only a few channels for information, these channels share all of the amplifier output power. As additional channels are added, the optical output power per-channel decreases.
In an optical communication network, the fiber medium is non-linear. This nonlinearity interacts with the dispersion of the fiber, and degrades the network performance. At high optical powers (e.g., more than 10 mW per channel), the optical signal experiences more distortion than at low optical powers (e.g., less than 1.0 mW per channel). Since the amplifiers of the network have a substantially constant output power level, the optical power per-channel at initial deployment may be much higher than the optical power per-channel in a fully loaded optical network. As a result of the initial high per-channel power and the system non-linearities, the network communication performance at initial deployment may be worse than the performance when the network is fully loaded.
Accordingly, there is a need for system and method for improving communication performance of an optical communication system operating with unutilized system channels.